1. Field of the Invention
The present invention is generally in the field of semiconductor devices. More particularly, the invention relates to fabrication of bipolar junction transistors.
2. Background Art
Bipolar junction transistors (BJTs) are commonly employed in amplifying and switching applications. As such, it is desirable to integrate BJT fabrication into complementary metal-oxide-semiconductor (CMOS) processes. A conventional BJT fabricated with a BiCMOS (Bipolar CMOS) process includes adjacent doped semiconductor regions having an NPN or PNP doping configuration. The adjacent doped regions constitute an emitter, a base, and a collector. In the BiCMOS process, also referred to simply as a “CMOS process” in the present application, fabrication of the conventional BJT is integrated with fabrication of a lateral metal-oxide-semiconductor field-effect transistor (MOSFET). The collector of the conventional BJT can be formed in a substrate. The base of the conventional BJT can be formed in the collector utilizing formation steps for a well of the lateral MOSFET in the substrate. Furthermore, the emitter of the conventional BJT can be formed in the base utilizing formation steps for a source and drain of the lateral MOSFET in the substrate.
However, the conventional BJT fabricated in the CMOS process has poor performance and is thus not suitable for high performance applications. One cause of the poor performance of the conventional BJT is that the emitter is necessarily much smaller than the base. As such, the conventional BJT has high emitter series resistance and low current conduction capability. Another cause of the poor performance of the conventional BJT is that its base-emitter junction is not well defined. As such, the conventional BJT has high base leakage current.
It would be desirable to provide for a BJT that can have improved performance over the conventional BJT. It would further be desirable that fabrication of such a BJT can be integrated with CMOS processes.